Total Restoration of a 1959 Superlead from 1973

[Matthews Guitars]

I'm going to drop that 10K to 8.2K. Because I have it. That should lower the voltages somewhat.

 

[Guitar-Rocker]

If you drop the 10k to a 8.2k, the resistance is less and the voltage increases

 

[Matthews Guitars]

I realize that, and I expected it. I wanted to get a sense of how much a resistance change varies the voltage,
which will let me calculate current draw by the voltage drop as well.

There's something really funky going on with the bias. The circuit is working. It IS grounded. I can vary the voltage at the 5.6K resistors by a fair amount. (38 to 50 volts, roughly.) But the power tubes don't seem to be drawing any current.

So, experimentally, changing the resistor values in the totem pole will change the screen grid voltage. We control bias via the control grid, but the screen grid plays a role as well. You can't ignore it.

And I'm wondering if something about the screen grid voltage is keeping the tube from drawing bias current. I have a certain amount of audio power but without cranking the amp I'll never know how much. I do think that the tubes are only partially on. If I run the bias pot from one end of travel to another, I see a SMALL change in screen current, but it's a matter of just about 2 MV. However this does change the level of any residual hum.

If the tubes are barely conducting, it stands to reason that the voltage dividers won't work correctly. They need a load in order to start seeing any real voltage drops.

 

[Matthews Guitars]

....and it helps to connect the plate supply voltage for the phase inverter to the correct spot! I ended up having to move that jumper anyway, which I had hoped to avoid doing. Now instead of an X on those jumpers, they're two parallel diagonals. I had to move it from the bottom to the top of the totem pole.

That solved my voltage issues. Now all the voltages are right where they're supposed to be according to the schematic.

But the bias system still isn't working right. I can control the bias voltage but the read bias current is still down between 0 and 2 mA.

 

[Trouble Free]

The circuit is working. It IS grounded. I can vary the voltage at the 5.6K resistors by a fair amount. (38 to 50 volts, roughly.) .

-38 to -50?

 

[Matthews Guitars]

Eventually I figured it out. I've got everything working properly now.

It was all related to that mess around the PI cap, the screen cap, and the associated wiring. At one point I had the screen tied to the low voltage side of the totem pole so it was running at 330 volts with a 470 volt plate. Moving that screen feed to the high side of the totem pole solved THAT. And making sure that the phase inverter plates are fed from the low voltage side of the totem pole was equally important.

Now for the wire dressing and cleanup. And I'm going to redo the filament string because it's kind of sloppy. I can do better.

I turned the PT around late last night so the wires all come out of the holes they're supposed to. I hadn't realized until yesterday that it was installed 180 degrees off. It'll be a clean wiring job after I've got everything trimmed and dressed.

 

[Matthews Guitars]

I have to say, I've learned a LOT from this rebuild. When I rebuilt the '73 Superlead, it went together very smoothly as it didn't vary from readily available documents at all, being an early ST1 board type. As a result, I didn't really learn all that much about how the amp really works.

But this one was super educational.

I now can point to any component in the amp and know what it is, where it is in the schematic, and its function. That doesn't mean I know every little nuance of it, but at least there are no BIG mysteries.

I've ordered up a new '69 spec headshell for it from Sourmash. And I'm now working on getting my first batch of real plexi reproduction panels made. (Actually they're lexan polycarbonate, which is stronger.)

I plan to play it for a while and explore its tonal range thoroughly before I do any mods to it and put that extra tube socket to work.

 

[Matthews Guitars]

It's done. Leads trimmed and dressed, tested and played up to and including full volume. Sound is similar enough to my '73 that I don't think there are any significant bugs in it.

The channel 1 volume doesn't go down quite to zero. Not sure if that's due to the volume pot or not. But it still gets quiet enough that it's not an issue under any conceivable circumstances.

 

[Matthews Guitars]

I meant to ask this some time ago. But forgot.

Being as this is a '69 Superlead, equipped with a grommeted power cord and no mains voltage selector, what power tube type would it have most likely been equipped with originally? KT-66 or EL-34?

My supply voltage is right around 470 volts with 120 volts AC input power. All voltages are well within specifications as per the print.

 

[Matthews Guitars]

I need another old ratted out Marshall to restore! Someone find me one! The rattier, the better!

 

[Matthews Guitars]

The photos:

 

[neikeel]

what power tube type would it have most likely been equipped with originally? KT-66 or EL-34? .

For 50 and 100w
El34 were stock, typically Mullards
KT66 stopped early 67
6550 late 70s early 80s in USA - better confirm that last bit!

 

[Matthews Guitars]

A hint of what's to come....I'm starting to make plexi replica panels now to complement my metalface panels.

My samples were cut with incorrect hole sizes but that doesn't keep me from testing the gold paint and the printing.

Yes, this is the wrong font. I know. But I'm just doing tests for now. I do have the right font, and since everything else is going according to plan, I'm starting plexi panel production in a week or two at most.

I'm using Lexan polycarbonate. Which is several times stronger than plexiglas.

 

[Matthews Guitars]

Quick question.

The .68 Mustard cap bypassing the 820 ohm resistor on V2....some amps have it, some don't.

Tonally speaking, what's the difference?

Should this '69 lead spec amp have that cap or not?

 

[neikeel]

Quick question.

The .68 Mustard cap bypassing the 820 ohm resistor on V2....some amps have it, some don't.

Tonally speaking, what's the difference?

Should this '69 lead spec amp have that cap or not?

Probably yes with an 820R cathode resistor and 47k/8ohm NFB

Later amps used 1k cathode resistor but no cap and 100k/4ohm NFB

 

[Matthews Guitars]

Tonally speaking, how is it going to alter the sound or response of the amp?

I'm also seeing that SOME amps stack a 10K resistor and an 8.2K resistor in the totem pole, while others stack two 8.2Ks. I'm not sure which is more
correct for this amp, either. I am getting 468 volts at the plates and the KT66s are biased to 70 percent value. Whatever that is. I confess I don't remember exactly
but I think it was 32 mA per tube.

The tonal goal is, basically, "vintage brown", a true Plexi tonality.

 

[stickyfinger]

Curious as what mA you measure on the coupling caps for V1 an tonestack.

 

[Matthews Guitars]

Next time I have the chassis out of the box I'll take measurements.

The current box is unshielded. I've got a new one coming from Sourmash for it. That one's shielded and I'm hoping that'll cut down on the residual noise. There is some white noise present even with the volumes all the way off. It's very low in level but it's there. It is most affected by the positions of Volume I and Volume II. And a little bit of 60/120 Hz hum is present as well. I may have to check some coupling caps for leakage. Volume II (bright channel)'s volume pot is a bit scratch though all pots are brand new. Maybe there's a little DC on there due to a leaky coupling cap. All things to check next time I've got it on the bench.

 

[neikeel]

Tonally speaking, how is it going to alter the sound or response of the amp?[?quote]

Upper mid boost (people have used a bigger cap like 300uF electro but IMO gives you mush, Friedman uses a 680nF and a largish electro in parallel there and if you had a hack amp to play with you could put in a 3 way switch, none, 680nF and say 100uF options.

I'm also seeing that SOME amps stack a 10K resistor and an 8.2K resistor in the totem pole, while others stack two 8.2Ks. I'm not sure which is more
correct for this amp, either.

The higher the value of those two resistors the lower the voltages downstream from PI to preamp, the 10k/8k2 is commonly used on amps with 490 on octal anodes. 8k2/8k2 is a 50w value with 420v on octal anodes. Depends on how brown you want your preamp. Bigger resistors= lower preamp voltage = less headroom and vice versa but at these values the differences are pretty minor.

I am getting 468 volts at the plates and the KT66s are biased to 70 percent value. Whatever that is. I confess I don't remember exactly, but I think it was 32 mA per tube.
The tonal goal is, basically, "vintage brown", a true Plexi tonality.

 

[stickyfinger]

Scratchy pots indicate DC likely. Unlikely that the Mustards are bad unless you damaged them soldering. I've used well over 100 NOS Mustards and never had a leaky cap.
If you have mA on the Mustards you need to pull the lead and measure the actual turret for a leaky board as was my thought behind my first question.